ONE OF THE MOST important recent medical breakthroughs has been the impact of gut flora on overall health. This dynamic mix of bacteria in the digestive tract may affect everything from obesity to asthma. It’s a topic that Reader’s Digest has been closely covering, including in my recent book 21-Day Tummy, a plan to shrink and soothe your stomach by reducing inflammation and balancing gut bacteria.

I’m also intrigued by this latest pioneering research: Gut microbes, new studies show, may even influence how happy or anxious we feel.

THE GUT-MOOD LINK
Anyone who’s ever experienced “butterflies” in his or her stomach knows that the gut and the brain are connected. But until recently, scientists hadn’t realized that the trillions of bacteria in our digestive tract may be driving the relationship.

One recent experiment suggests that you can colonize calm. Canadian researchers gave healthy mice a cocktail of antibiotics, which alter the makeup of GI-tract bugs. Over two weeks, some animals became more anxious and some less, depending on which drugs they received. In another study, when gut bacteria from calm mice were transferred to anxious mice, the jittery critters seemed less nervous.

Everyday foods may also affect the composition of gut bacteria, and in turn, brain chemistry. UCLA gastroenterologist Kirsten Tillisch, MD, recently had healthy women eat a yogurt rich in certain strains of “good” probiotic bacteria. Two control groups ate yogurt without such bacteria or ate nothing. Brain scans of the probiotic-yogurt eaters indicated changes in regions that could be associated with a less-anxious response in fearful or stressful situations compared with the control groups.

R. Kikuo Johnson for Reader’s Digest

Scientists are even exploring whether gut microbes might treat some brain disorders. A new study in Cell found that mice with features of autism given a type of bacteria in healthy human GI tracts exhibited less autism-like behavior. In a recent case report, a Boston psychiatrist says a course of certain probiotics and antibiotics helped relieve a patient’s obsessive-compulsive disorder and ADHD.

“We tend to focus on how your mood affects your body from the top down, not the bottom up,” says Dr. Tillisch. “Now we know that the gut affects how your brain responds to the environment—it’s a remarkable change in thinking.”

HOW BACTERIA TALK TO THE BRAIN
Scientists aren’t sure how gut flora affect brain chemistry. One theory is that the bacteria access pathways along the vagus nerve, the main highway in the nervous system that links the brain to the gut. Another is that the bacteria influence the immune system, releasing chemicals that affect conditions like depression. Finally, researchers believe that bacteria produce or affect the metabolism of chemicals like serotonin or dopamine, which alter brain function.

CULTIVATE A HAPPIER GUT
Dr. Tillisch told me that patients always ask which probiotics will lift their mood. Her answer: “We don’t know yet, but we hope to one day.” In the future, patients might receive probiotics or fecal transplant—in which bacteria from the gut of a healthy person are transferred to the gut of a sick one—to prevent or treat mental health issues. But these approaches are years away from prime time. Until then, a diet rich in fruit and veggies (whose fiber nourishes certain bacterial strains) helps cultivate better-balanced microbes than a typical Western diet high in animal and processed products. Avoiding unnecessary antibiotics, particularly in early childhood, may also establish healthier gut bacteria.